Emission from Charge Transfer States

Veldman et al. [77] investigated the electric field-induced quenching of the emission from charge transfer states in a fluorine copolymer PCBM blend (Figure 4.16). There are three especially remarkable aspects of the results of these measurements. 

It should be noted that broad emission spectra are also observed in some cases for molecular dimers and larger aggregates, e.g., in the case of naphthalene [11]. The excited state in these homogeneous clusters is characterized as an excimer, and the interaction between the two components is mainly due to exchange interactions, although the contribution from charge-transfer states such as A A " is also important [12, 13]. 

M.L) it may be a MMCT state (see text). The arrow indicates the nonradiative transition from the charge-transfer state d to the ground state, so that c b and c - a emission is quenched... 

When photoluminescence spectra were recorded for a Ti(OSi(CH3)3)4 model compound, upon excitation at 250 nm only one emission band was detected (at 500 nm), which was assigned to a perfect closed Ti(OSi)4 site. The excitation of these species is considered to be a LMCT transition, 02 Ti4+ —<> (0-Ti3+), and the emission is described as a radiative decay process from the charge transfer state to the ground state, O Ti3+ — 02 Ti4+. Soult et al. (94) also observed an emission band at 499 nm, which they attributed to the presence of a long-lived phosphorescent excited state. The emission band at 430 nm of TS-1 was tentatively assigned to a defective open Ti(OSi)3(OH) site (49). 

When the emissive state is a charge transfer state that is not attainable by direct excitation (e.g. which results from electron transfer in a donor-bridge-acceptor molecule see example at the end of the next section), the theories described above cannot be applied because the absorption spectrum of the charge transfer state is not known. Weller s theory for exciplexes is then more appropriate and only deals with the shift of the fluorescence spectrum, which is given by... 

The formation of charge transfer complexes between N,N-dimethylaniline or N,N-diethylaniline and Cspectroscopic studies, also in view of their potential optical and electronic applications. Even if the spectroscopic properties of Cgo, C70 are complicated by the presence of aggregates in room temperature solutions, the emissions from the excited state charge transfer complexes between fullerenes and iVjV-dialkylani lines are strongly solvent-dependent exciplet emissions are observed in hexane, but in toluene they are absent145. 

A relevant result here is that the emissions of the relaxed and unrelaxed DMABN charge-transfer states have the same spectral shape the instantaneous spectral distributions deduced from the decays i( v, t) show no significant time dependence of their contours, which differ only slightly from that of the stationary distribution (Fig. 2.22). 

Because of the spectral relaxation due to the appearance of a high dipole moment in the charge-transfer state, the dynamics of the TICT state formation has been studied by following the fluorescence rise in the whole A band. In Fig. 5.6 are plotted, in the 10 ns time range, the experimental curve iA(t) at -110°C in propanol (tj = 1.5 x 103 cp) and the decay of the B emission at 350 nm. The solid curve representing the evolution of the TICT state expected in a constant reaction rate scheme shows a slower risetime with respect to that of the recorded A emission. To interpret the experimental iA(t) curves, the time dependence of the reaction rate kliA(t) should be taken into account. From the coupled differential equations for the populations nB(t) and nA(t) of the B and A states (remembering that the reverse reaction B <—A is negligible at low temperatures) ... 

A comparison of the absorption and fluorescence properties of 3,5-diaryl-l//-pyrrole-2-carboxylic acid ethyl esters reveals marked effects of/i-dimethylamino and o-methyl substituents on the phenyl group at C-3 and suggests that in the presence of these substituents emission occurs either from a charge transfer state or from a combination of charge transfer and locally excited states <2005NJC1258>. 

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